<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        *{
            padding:0;
            margin:0;
        }
        .site-nav{
            height: 30px;
            background-color:#ccc;
        }
        .top-banner{
            background-color:blue;
        }
        .w{
            width: 1210px;
            height: 80px;
            margin:0 auto;
            background-color:pink;
            position:relative;
        }
        .search{
            width: 1210px;
            height: 80px;
            margin:0 auto;
            background-color:red;
        }
        a{
            display: inline-block;
            text-decoration:none;
            width: 25px;
            height: 25px;
            position:absolute;
            top:10px;
            right:10px;
            text-align:center;
            line-height:25px;
            background-color:#000;
            color:#fff;
        }
    </style>
</head>
<body>
    <div class="site-nav"></div>
    <div class="top-banner" style="opacity: 1;">
        <div class="w">
            <a href="#">×</a>
        </div>
    </div>
    <div class="search"></div>
    <script>
        //点击关闭按钮，先让top-banner这个盒子透明度变为0，紧接着display：none;

        var topBanner=document.getElementsByClassName("top-banner")[0];
        var a=topBanner.children[0].firstElementChild||topBanner.children[0].firstChild;
        var timer=null;
        a.onclick=function(){
            timer=setInterval(function(){
                topBanner.style.opacity-=0.1;
                if(topBanner.style.opacity<0){
                    topBanner.style.display="none";
                    clearInterval(timer);
                }
            },50)
        }
    </script>
</body>
</html>